An extended time is also given on the workings of immunology and how the immune systems of living beings respond to parasite infection, along with the methods that bodily functions use to counteract and potentially kill invading microorganisms.

[2] In a review for Science, Albert O. Bush pointed out how Zimmer creates a writing style that is written with "clarity, conviction, and seemingly without prejudice" and that while the "purist will find the odd mistakes, oversights, and minor errors of fact", these are "insignificant" and do not remove from Parasite Rex's "overall quality or, more importantly, its focus and take-home message.

[8] A review in The American Biology Teacher by Donald A. Lawrence labeled the book as a "splendid overview of current knowledge about parasites" and praised the extensive Notes, Literature Cited, and Index sections.

[10] BlueSci writer Harriet Allison summed up the book as one where Zimmer "manages to weave just enough easily understandable science into each chapter in order to create an engrossing and squirm-inducing story that will have you hooked until the end".

[11] Kirkus Reviews stated its acclaim for the "vivid detail" given to the lifestyles of parasites, calling the book an "eye-opening perspective on biology, ecology, and medicine" and "well worth reading".